There is no reason for there not to be a fleet constitution refit retrofit. The defiant, Nova, and Jem bug are all smaller ships which have fleet stats, the connie can get them too.

http://sto-forum.perfectworld.com/sh....php?t=1313421
The Somraw, K'tinga, D'Kyr, D7, Kumari, Xindi carrier, Xindi escort, and the T'Varo are all older than the Constitution Refit and yet they are tier 5. The Constellation is made up primarily of Connie refit parts and it is tier 5, there is no logical reason whatsoever for the no tier 5/6 connie rule.